在C中多次声明一个函数有什么意义？

Question

I've seen a couple of times that the prototype declaration of a function in the header was literally repeated in the c-file.

It is possible to declare a function more than one time in C - but what sense does it make? Is it just for better readability or is there some deeper insight which I am missing?

Answer 1

It is possible. It doesn't make any sense.

But it does not cause any harm either. You can declare a function as many times as you like, but each such declaration must be identical to the others. So it is pointless to do so. As someone suggested, maybe it is a copy/paste error.

You can only have one function definition though, which should always be in the c file.

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This is how you should do it:

• Function declarations which is part of the caller's interface should be in the h file, and there only.

• Function declarations of local (private) functions that are only available from inside the c file itself, should be in the c file, and there only. Such functions should be declared and defined as static .

Answer 2

Duplicate function declarations serve no useful semantic purpose, but they may arise for historical reasons, because of local coding conventions, or for some other reason.

For example, it may be a local coding convention that every function in each source file is prototyped at the beginning of that file. That has some minor practical utility, such as serving as a manifest of the functions defined in each file, and making the functions inside each file able to ignore any concern about whether other functions in the same file are declared in a header.

Additionally, multiple declarations of the same function or object don't necessarily have to be identical , they only need to be compatible . Under some circumstances it may makes sense to provide a less specific prototype in a header and a more specific one in the source file containing the function definition (which itself serves as yet another declaration).